The present invention relates to a low-loss, push-pull current-fed DC-DC converter for yielding DC voltage.
FIG. 1 is a circuit diagram of a conventional push-pull current-fed DC-DC converter. An input capacitor 6 is connected across an input DC power supply 5. One end of DC power supply 5 is connected to respective first ends of a diode 7 and the primary winding a choke coil 8. The other end of the diode 7 is connected via a secondary winding, or feedback winding, of the choke coil 8 to the other end of the DC power supply 5. The primary winding of the choke coil 8 is connected at the other end thereof to the junction of primary windings 2 and 3 of a push-pull transformer 110, and the other ends of the primary windings 2 and 3 are, in turn, connected to the other end of the DC power supply 5 via MOSFETs 9 and 10 serving as main switching elements The MOSFETs 9 and 10 include parasitic diodes 11 and 12, respectively. Across a secondary winding 4 of the transformer 110 there is assumed a capacitor 13 which represents a distributed capacitance of the secondary winding 4. The output of the secondary winding 4 is connected to the input side of a bridge circuit composed of rectifying diodes 14 to 17. Connected to the output side of the bridge circuit are a smoothing capacitor 18 and a load resistor 19.
FIG. 2 shows waveforms occurring at respective parts of the converter depicted in FIG. 1. The switching elements 9 and 10 are supplied at their gates with gate signals V.sub.G1 and V.sub.G2 of the same frequency f and the same ON-OFF ratio but displaced 180 degrees apart in phase. When the switching element 9 conducts and current I.sub.D1 flows therethrough at time t.sub.0, current I.sub.L flows from the choke coil 8 into the primary side of the transformer 110 at the same time. As a result, a voltage -V.sub.T is created in each of the primary windings 2 and 3 of the transformer 110, and current flows in the secondary winding 4, charging the distributed capacitance 13. When the voltage across the capacitance 13 (corresponding to voltage V.sub.T on the primary side) exceeds the voltage V.sub.out of the output capacitor 18, the rectifying diodes 14 and 17 are turned ON, charging the output capacitor 18 by current I.sub.Dr. The current I.sub.Dr shown in FIG. 2 represents a rectified current which flows through either of the pair of diodes 15 and 16 or the pair of diodes 14 and 17 in FIG. 1, although the current flow is indicated by arrows in FIG. 1 beside only the pair of diodes 14 and 17.
When the switching element 9 is turned OFF at time t.sub.3, the diode 7 is immediately turned ON and conducts for a period (t.sub.3 -t.sub.4) during which the switching elements 9 and 10 are both OFF, by virtue of the continuity of the current flowing through the choke coil 8 for the period (t.sub.0 -t.sub.3) during which only the switching element 9 was ON; so that current flows via a route [feedback winding of the choke coil 8 .fwdarw.diode 7 .fwdarw.input power supply 5], thus feeding back the excitation energy of the choke coil 8 to the input power supply 5. Similarly, since the exciting current for the transformer 110 cannot flow to the primary side thereof during this period (t.sub.3 -t.sub.4), a voltage is generated in the transformer winding when the exciting current flows to the secondary side. The exciting current of the transformer 110 acts as a discharging current of the distributed capacitance 13. As a result of this, voltage V.sub.T of the transformer 110 gradually approaches zero. Next, when the switching element 10 is turned ON at time t.sub.4, current I.sub.D2 flows therethrough owing to voltage V.sub.DS2 applied across the switching element 10 immediately prior to its conduction, and at the same time current I.sub.L flows into the primary side of the transformer 110 from the choke coil 8. In consequence, current flows across the secondary winding 4 as is the case with the above, charging the distributed capacitance 13 in a reverse direction. When the voltage across the capacitance 13 exceeds the output voltage V.sub.out across the capacitance 18 at time t.sub.6, the rectifying diodes 15 and 16 are turned ON to cause a current I.sub.Dr therethrough, by which the output capacitor 18 is charged.
In this way, a voltage which is a multiple of the turns ratio of the transformer 110 is obtained, in the same form as the waveform V.sub.T shown in FIG. 2, on the secondary side of the transformer 110, and this voltage is rectified and output from the DC-DC converter as the output voltage V.sub.out.
Such a conventional push-pull current-fed DC-DC converter as shown in FIG. 1 employs, in the transformer 110, a no-gap core 100 as depicted FIG. 3, and its excitation inductance is so large that the rate of exciting current contained in the current flowing in the primary winding (2 or 3) of the transformer is very low. The value of this exciting current represents the value of energy stored in the excitation inductance of the transformer. In the case of the transformer of the prior art converter, the energy stored in the excitation inductance is discharged to the secondary side in the first half of the ON period of the main switching element and is stored in the latter half of the ON period. Even if the exciting current is large, its energy will not be entirely lost, but since it is partly consumed as an increase in the copper loss in the transformer windings, it is usually considered preferable that the value of the exciting current be small. However, the present inventors' analyses have revealed that where a transformer of a high turns ratio is employed in the conventional converter for the purpose of generating a particularly high voltage, an increased distributed capacitance of the transformer would cause an increase of the conversion loss because of the small exciting current. Next, a description will be given, with reference to FIGS. 1 and 2, of the mechanism of the increase in the loss.
In the period during which the main switching element 9 is ON (t.sub.0 -t.sub.3 in FIG. 2), the distributed capacitance 13 on the secondary side of the transformer 110 is charged by a voltage which is negative at the side of the transformer winding marked with the black dot, relative to the other side thereof. When the main switching element 9 is turned OFF at t.sub.3, the exciting current of the transformer 110 reduces the charges stored in the distributed capacitance 13 and hence decreases its voltage, because the exciting current flows from the secondary winding 4 at the side indicated by the black dot. The variation in the voltage across the distributed capacitance 13 is similar to the variation in the primary winding voltage V.sub.T. Where the charges stored in the distributed capacitance 13 are not reduced to zero until time t.sub.4, the turning ON of the main switching element 10 causes the charges in the distributed capacitance 13 to constitute a short-circuit current I.sub.S which is discharged via a route [primary winding 3 of the transformer 110 .fwdarw.main switching element 10 .fwdarw.parasitic .fwdarw.diode 11 of the main switching element 9 .fwdarw.winding 2], resulting in the loss of the energy stored in the distributed capacitance 13 until just before time t.sub.4. After the short-circuit current period (t.sub.4 -t.sub.5) the distributed capacitance 13 is charged by current I.sub.L of the boosting choke coil 8 and becomes positive at the black-dot side of the transformer winding. As will be seen from the above, the less the exciting current is, the more the voltage of the distributed capacitance 13 remains undissipated just before time t.sub.4 (see waveform V.sub.T in FIG. 2), causing an increase in the loss. Incidentally, in a converter for creating high voltage through use of a transformer having a higher turns ratio, n.sub.T (=N.sub.T2 /N.sub.T1), of the number of turns N.sub.T2 of the secondary winding 4 to the number of turns N.sub.T1 of the primary winding 2 (or 3), since the value of the distributed capacitance as viewed from the primary side increases correspondingly, the loss by the distributed capacitance will increase; further, the loss increases as the switching frequency, i.e. the conversion frequency f rises.
The above phenomenon will occur also in the case of employing EI or EE cores in the transformer of the push-pull current-fed DC-DC converter, because the cores are joined together with no gap therebetween, providing a large transformer inductance.
In addition to the loss by such a short-circuit current, the prior art has presented a problem that an increase in the conversion frequency f causes an abrupt increase in the loss, since the excitation inductance of the choke coil 8 has heretofore been determined taking only input current ripples into account.